Polyolefin microporous membranes are used as microfiltration membranes, separators for batteries, separators for fuel cells, separators for capacitors, and the like. Of these, polyolefin microporous membranes are suitably used as separators for lithium ion batteries due to their shut-down characteristics and excellent pin puncture strength of the membranes.
In recent years, the application of lithium ion batteries to automobiles, equipment for outdoor use, and the like has been investigated, and there is a need for larger batteries. Accordingly, there is also a demand for wide separators, with 100 mm or greater in width for example, which are incorporated into such batteries. However, polyolefin microporous membranes used in separators have a tendency to curl markedly as the width increases. Such curling in a separator impairs the processability, and it becomes difficult to obtain a roll of a polyolefin microporous membrane with a good winding. Meanwhile, increases in the width or length, decreases in membrane thickness, and increases in the production speed of a microporous membrane roll product have been investigated in order to improve productivity. However, these changes may lead to the manifestation of the problems described above as well.
Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2011-140633A) discloses a polypropylene film roll, which has excellent productivity, exhibits excellent battery characteristics when used in a separator of a storage device, and has enhanced processing suitability in the battery production process. However, the obtained film was not able to provide sufficiently high strength and high permeability simultaneously.
Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2009-132904A) discloses a film prepared by stretching a polyethylene gel-like sheet having a weight average molecular weight of 3.8×105 by 8.5 times in the MD direction and by 5 times in the TD direction sequentially, washing and drying off the membrane-forming solvent, and then stretching the sheet by 3.0 times in the MD direction and by 1.2 times in the TD direction in a restretching process. Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2010-24463A) discloses a film prepared by removing part of a solvent from a gel-like sheet made of ultra-high molecular weight polyethylene having a weight average molecular weight of 2.0×106 and high-density polyethylene having a weight average molecular weight of 3.5×105 and then stretching the sheet by 5 times in the MD direction and by 10 times in the TD direction sequentially. However, both of these inventions were not able to achieve excellent pin puncture strength and air permeability simultaneously. Also, they were not able to improve the appearance (wrinkles, winding deviation) sufficiently when formed into a roll.
Patent Document 4 (Japanese Unexamined Patent Application Publication No. 2012-221889A) describes a multilayer, porous membrane having excellent resistance against curling. However, the disclosure only describes the reduction of curling in a multilayer, porous membrane obtained by forming a porous layer on the surface of a polyolefin microporous membrane, and does not describe the reduction of curling in the polyolefin microporous membrane itself, which serves as a base material.